1. Field of the Invention
The invention relates to a transflective display device, and more particularly to a transflective liquid crystal display (LCD) device with an optical supplement structure for improving viewing angle, increasing light recycling rate and reducing thickness thereof.
2. Description of the Related Art
Liquid crystal display (LCD) devices are usually classified as transmissive type or reflective type according to the difference in their display light source. The transmissive type LCD device uses a back light module, in which the light is incident to an LC layer and is absorbed or passes through the LC layer, thus the disadvantages of faded color and reduced contrast ratio occur under a natural light source or an artificial exterior light source. Conversely, the reflective type LCD device relies on ambient incident light from an exterior light source, and offers superior performance and high contrast under outdoor sunlight. Also, because of its low power consumption, the reflective type LCD devices are primarily employed in portable display products. The quality of reflective type LCD devices, however, suffers when the exterior light source is obscured, and it is comparatively difficult to achieve high resolution for a full color display. Accordingly, transflective LCD devices have been developed to compensate for the previously mentioned disadvantages and combine the advantages of reflective and transmissive LCD devices. The transflective LCD device can use well known active driving processes, such as amorphous silicon thin film transistors (a-Si TFTs) or low temperature polysilicon (LTPS) TFTs, and is applicable to low power products.
FIG. 1 is a cross-section of a conventional transflective LCD device. A transflective LCD device 10 comprises an upper substrate 12, a lower substrate 14 and an LC layer 16 interposed therebetween. Adjacent to the inner surface of the upper substrate 12, opposing the LC layer 16, lies a color filter and a common electrode layer 18. On the outer surface of the upper substrate 12, a first quarter-wave plate (QWP) 20I, a first half-wave plate (HWP) 22I and a first polarizer 24I are successively formed. The first HWP 22I has an optical retardation of xcex/2, and the first QWP 20I has an optical retardation of xcex/4, in which xe2x80x9cxcexxe2x80x9d indicates a wavelength of the incident light.
On the inner surface of the lower substrate 14, opposing the LC layer 16, a transparent electrode layer 26, a passivation layer 28 and a reflective electrode layer 30 are successively formed, in which the transparent electrode layer 26 and the reflective electrode layer 30 act together as a pixel electrode. Also, an opening 29 is formed to penetrate the central portions of the reflective electrode layer 30 and the passivation layer 28, thus the exposed portion of the transparent electrode layer 26 serves as a transmissive area T of the pixel electrode, and the overlapped portion between the reflective electrode layer 30 and the transparent electrode layer 26 serves as a reflective area R of the pixel electrode. On the outer surface of the lower substrate 14, a second QWP 20II, a second HWP 22II and a second polarizer 24II are successively formed. The second HWP 22II has an optical retardation of xcex2, and the second QWP 20II has an optical retardation of xcex4. Additionally, a backlight device 32 is arranged adjacent to the second polarizer 24II.
Operation of the transflective LCD device 10 is described in the following. First, in reflective mode, external incident light is reflected from the reflective electrode layer 30 (the reflective area R of the pixel electrode), and is directed toward the upper substrate 12. At this point, when electrical signals are applied to the reflective electrode layer 30 by a switching element (such as a TFT device), the arrangement of LC molecules in the LC layer 16 varies and thus the reflected light is colored by the color filter, thereby displaying a color image. Second, in the transmissive mode, the light emitted from the backlight device 32 passes through the opening 29 (the transmissive area T of the pixel electrode). At this point, when the electrical signals are applied to the transparent electrode layer 26 by the switching element, the arrangement of LC molecules in the LC layer 16 varies and thus the light passing through the LCD device 10 is colored by the color filter, thereby forming a color image.
The object of forming the retardation films including the first QWP 20I, the second QWP 20II, the first HWP 22I and the second HWP 22II on both substrates 12 and 14 is to expand the optical compensation effect through the broad-wavelength light band. Also, in one pixel area, the LC layer 16 has a first cell gap over the reflective area R and a second cell gap over the transmissive area T, thus the phase retardation in the transmissive area T is twice the phase retardation in the reflective area R. The difference in the phase retardation between the reflective area R and the transmissive area T, nevertheless, impedes the retardation films to achieve the accurate optical compensation. Accordingly, based on the dual cell gaps design, reducing the cell thickness of the transflective LCD device 10 and reducing the thickness of each retardation film are considered. Moreover, the first QWP 20I and the second QWP 20II limit the viewing angle within the transmissive area T, thus a novel structure to solve the problem of narrowed viewing angle is called for.
The light recycling effect between the backlight device 32 and the reflective area R is concerned with the optical structure including the QWPs 20I and 20II and HWPs 22I and 22II. FIG. 2 is a cross-section illustrating the light recycling effect between the backlight device 32 and the reflective area R. When a first incident light 33 emitted from the backlight device 32 passes through the second polarizer 24II, the second HWP 22II and the second QWP 20II, the first incident light 33 is weakened and becomes a second incident light 34. When directed toward the upper substrate 12, the second incident light 34 is reflected from the reflective electrode layer 30 to form a first reflective light 35. After passing the second QWP 20II, the second HWP 22II and the second polarizer 24II, the first reflected light 35 is further weakened and becomes a second reflected light 36. Accordingly, the incident light and the reflected ight completely pass through the second QWP 20II and the second HWP 22II twice, and are mostly absorbed and weakened causing the second reflected light 36 to be extremely weak and incapable of being recycled. Thus, the light recycling rate is too low to provide adequate illumination, and a greater power is required to increase the light intensity of the backlight device 32 in order to improve the luminescent property of the transflective LCD device 10.
Accordingly, an object of the present invention is to provide a transflective display device with an optical supplement structure to achieve a smaller size, a thinner profile, and a lower cost.
Another object of the present invention is to provide a transflective display device with an optical supplement structure to achieve superior display performance at a wide viewing angle.
Another object of the present invention is to provide a transflective display device with an optical supplement structure to increase the light recycling rate.
Another object of the present invention is to provide a transflective display device with an optical supplement structure to achieve greater brightness and higher resolution.
According to the object of the invention, a transflective LCD device has an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. A reflective electrode layer is formed overlying the inner surface of the lower substrate to serve as a reflective area of a pixel electrode. A transparent electrode layer is formed overlying the inner surface of the lower substrate, in which the transparent electrode layer not covered by the reflective electrode layer serves as a transmissive area of a pixel electrode. A first polarizer is formed overlying the outer surface of the upper substrate. A second polarizer is formed overlying the outer surface of the lower substrate. An optical compensation plate is formed between the second polarizer and the lower substrate.